1. Field of the Invention
The present disclosure relates to geothermal energy production and particularly to apparatuses and methods for supporting Enhanced Geothermal Systems (EGS).
2. Description of the Related Art
Geothermal energy is an important part of the nation's renewable energy initiative. FIG. 1 illustrates a simplified schematic of a geothermal plant that generates electricity for the electrical grid. A working fluid (F) such as water is transferred with a pump 100 down into the hot rock formations through an injection well 102, where it absorbs heat energy from the fractured rock formation. The heated working fluid (F) is then pumped to an energy conversion plant 104 through a production well 106. Depending on the fluid's (F) temperature, it may directly be used to power a turbine or may be used to heat a secondary working fluid, which, in turn, is used to power a turbine. The turbine is coupled to a generator through a common shaft (not shown), to generate electricity for the electrical grid 108. The cooled working fluid (F) is then injected with the pump 100 back into the hot rock geothermal reservoir through the injection well 102 to sustain the process. Geothermal energy generation is considered a green technology, because little or no greenhouse gases are emitted into the atmosphere and the energy source is renewable.
An Enhanced Geothermal System (EGS) is a man-made reservoir, created where there is sufficient underground hot rock but insufficient or little natural permeability or working fluid saturation in the rock. EGS expands the geothermal energy domain into much deeper rock deposits by exploiting natural and artificial fracture systems/networks within rock mass. Maintaining and/or creating such facture networks in complicated geological environments are critical to the successful development and long-term sustainability of the EGS. The EGS targets a huge energy source that amounts to 500 GWe in the western U.S. and 16,000 GWe in the entire U.S. Several demonstration projects are undergoing in the U.S. to validate different reservoir stimulation techniques. The ultimate reservoir will have a flow rate of 60 kg/s, a lifetime of 30 years along the drilling systems down to 10,000 meters deep at 374 Degrees Celsius.
EGS reservoir stimulation technologies currently are adapted from the oil and natural gas industry including various hydrofracking methods with or without chemical additives. A potential drawback of using hydrofracking techniques is the lack of effective control in the creation of large fractures, which could result in by-pass of targeted fracture network or even fault movement in the rock formation. The loss of hydraulic medium can reduce heat exchange efficiency and increase the cost of the development of EGS. The use of chemicals along with the unpredictable fault movement may also adversely impact the environment.
Cavitation is the process of the formation of vapors, voids, or bubbles due to pressure changes in a liquid flow as schematically illustrated in FIG. 2. The pressure wave propagation 200, and eventual collapse of the bubbles 202 can cause local pressure changes in the liquid, which can be transmitted to a target rock surface 204 either in the form of a shock wave 206, or by micro jets 208, depending on the bubble to surface distance. Pressure greater than 100,000 psi has been measured in a shock wave 206 resonating from cavitating bubbles 202. It is generally understood that the cycle of formation and collapse of the bubbles that occurs, often at a high frequency, can generate dynamic stress on the surfaces of objects. Ultimately, the dynamic stress can contribute to the fatigue of the target surface, including micro-cracks that form and coalesce on the surface 204, eventually leading to material removal known as cavitation damage.
What are needed are apparatuses and methods for generating a pulse-pressure cavitation technique (PPCT) for use in Enhanced Geothermal Systems (EGS) and oil and gas wells.